1. Field of the Invention
The present invention relates to a printed board machining apparatus for machining workpieces by a tool while pressing down the peripheral part of the workpiece to a table by a pressure foot.
2. Related Background Art
FIG. 4 is a schematic perspective view showing an outside view of a prior art printed board hole driller, i.e., an exemplary printed board machining apparatus, and FIG. 5 is a front section view of an edge of a spindle and a pressure foot of the printed board hole driller shown in FIG. 4.
As shown in FIGS. 4 and 5, the printed board hole driller 14 is constructed so that a table 1 therein moves in the direction of an arrow X by an X-axis motor 2 on a track 4 fixed on a bed 3. A column 5 is fixed to the bed 3 so as to straddle the table 1. A cross slide 6 is arranged to move in the direction of an arrow Y on a track 9 fixed on the column 5 by a Y-axis motor 7 and a ball screw 8. A housing 10 is arranged so as to move up and down on a track 13 fixed to the cross slide 6 in the direction of an arrow Z by a Z-axis motor 11 and a ball screw 12.
The housing 10 supports a spindle 21 that rotably supports a drill 20, i.e., a tool. The housing 10 also supports a pressure foot body portion 22 that engages with the edge portion of the spindle 21 through a cylinder not shown so as to move up and down in the Z-axis direction. The pressure foot body portion 22 is provided with a bush 23 at the end thereof. A drill through opening 29 through which the drill 20 moves in and out is made at the axial center position of the bush 23. A part in which the pressure foot body portion 22 is combined with the bush 23 will be referred to as a pressure foot 32 hereinafter.
A groove 24 connecting an outer peripheral part of the bush 23 with the drill through opening 29 is made at the lower end of the bush 23. A space 25 made by the spindle 21 and the pressure foot 32 is connected to a dust collector 27 via a pipe 26. ACNC unit 28 controls the motors of the respective axes.
A plurality of printed boards 30, i.e., workpieces, are stacked and are fixed on the table 1 together with a stiffening plate 31 placed at the top thereof. The stiffening plate 31 is made from an aluminum plate having a thickness of about 0.5 mm.
In drilling the stiffening plate 31 and the printed boards 30, the printed board hole driller 14 lowers the pressure foot 32 by an air cylinder not shown, turns on the dust collector 27 to vacuum the space 25 and lowers the spindle 21 to a predetermined height after positioning the drill 20 at a drilling position. Then, the printed board hole driller 14 presses down the stiffening plate 31 and the printed boards 30 to the table 1 by the pressure foot 32 to drill the stiffening plate 31 and the printed boards 30 by the drill 20.
When the drilling ends, the printed board hole driller 14 raises the pressure foot 32 and the spindle 21 to a position where the lower end of the pressure foot 32 (lower end of the bush 23) does not interfere the stiffening plate 31 and moves the pressure foot 32 and the spindle 21 in the horizontal direction together with the drill 20 to position the drill 20 at a next machining position.
Because the prior art printed board hole driller 14 is constructed so as to cut into the stiffening plate 31 by the drill 20 while pressing down the stiffening plate 31 and the printed boards 30 to the table 1 by the pressure foot 32 as described above, it permits an accuracy of machining position to be improved. Further, because the prior art printed board hole driller 14 is constructed so as to be able to collect work chips produced in drilling the workpieces immediately within the dust collector 27 by air flowing into the space 25 through the groove 24 as shown in FIG. 5, it permits holes to be made accurately.
By the way, it is desirable to keep the position of the edge of the drill 20 as close as possible to the stiffening plate 31 (hereinafter referred to as a ‘standby position’) in moving the drill 20 to the next machining position in order to improve the drilling efficiency of the printed board hole driller 14. However, because the position of the edge of the drill 20 in connection with the lower end of the pressure foot 32 is set in advance, the lower end of the pressure foot 32 is brought closer to the stiffening plate 31, and a distance L1 between the stiffening plate 31 and the pressure foot 32 is shortened when the position of the edge of the drill 20 at the standby position is brought closer to the stiffening plate 31.
However, when the distance L1 is shortened too much, there is a case when the stiffening plate 31 is lifted up because it is suctioned to the side of the pressure foot 32 by the dust collecting force of the dust collector 27 as shown in FIG. 5. Then, the stiffening plate 31 might be damaged if it collides with the lower end of the pressure foot 32. When the damaged part is the part to be drilled next, drilling may not be done accurately at the predetermined position or the drill may be broken as the edge of the drill 20 may slide when it abuts the stiffening plate 31.
As a printed board hole driller dealing with such a problem, Japanese Patent Laid-open No. Hei.3-3713 has proposed one that pressurizes a stiffening plate and printed boards by compressed air while lifting up a pressure foot from the stiffening plate by forming an air film between the lower face of the pressure foot and the stiffening plate by blowing compressed air from a port made at the lower face of the pressure foot in drilling the printed boards. It allows to shorten the distance L1 and to prevent the interference between the pressure foot and the stiffening plate, thus preventing the stiffening plate from being damaged.
However, the prior art printed board hole driller has required a large amount of compressed air to support the pressure foot. Further, the pressurizing force fluctuates considerably, and the pressurization of the stiffening plate and the printed boards has been unstable as compared to the case of abutting the pressure foot directly to the stiffening plate.